Q: That is to say, if I have a 500 Watt PSU, will it constantly draw 500 Watts when the computer is in use, or will it draw just enough to power the attached hardware? Or, in other words, is there any benefit to the environment and my electricity bill to get the minimum wattage I need rather then allowing room for expansion in the future.
A: The overall wattage (500W, in your case) is the maximum the PSU is rated for. The components inside of your case dictate how much electricity is used. In other words, a 350W and 750W PSU will use the exact same amount of energy ( all other things being equal). Large power supplies tend to last longer than lower-rated ones as larger PSUs do not have to work as "strenuously"...this prolongs component life. Good luck!